1. Field of the Invention
The present invention relates to a photomask for forming a resist pattern and fabrication method of the photomask and a resist-pattern forming method using the photomask.
2. Description of the Related Art
Lithography is used in the field of micro-fabrication, as well known. In the usual lithography, the planar form of a photomask pattern is once transferred onto resist. By etching process, the pattern on the resist is further transferred onto a layer-to-process, thus obtaining an etched pattern geometrical similar to the photomask pattern.
However, there is a recent proposal on an attempt to etch a layer-to-process three-dimensionally through use of lithography.
For example, in the optical-transmission system in which the signal light is propagated through an optical fiber from a semiconductor laser to a light-receiving diode, the emission light from the semiconductor laser is focused by a lens. By reducing the spot diameter of the emission light on an incident plane of the optical fiber nearly to a core diameter of the optical fiber, the transmission efficiency can be improved.
In such an application, a micro lens having a diameter of approximately 250 μm is used as exemplified in a conventional fabricating method, e.g. see Japanese Patent Applications Kokai No. H8-166666 (JP-A-H8-166666) and No. 2003-177507 (JP-A-2003-177507).
JP-A-H8-166666 discloses a fabrication method including the steps as follows. First, resist is applied onto a substrate that a lens is to form. On a transparent mask substrate, resist is exposed to light through use of a photomask formed with a shade film having concentric shade areas separated by spaces which are transparent areas. Then, the exposed resist is developed to obtain a resist pattern. Thereafter, the resist pattern is transferred by dry etch onto the substrate, thus obtaining a lens.
In the photomask used, the diametric pitch of the concentric shade areas is smaller than the length of a resolution limit exhibited by an optical system of an exposure equipment. Accordingly, the light transmitted through the photomask does not possess a sufficient contrast in terms of resolution for the resist. The dimension between adjacent spaces, i.e. spacing between shade areas, increases in a diametric direction of from the concentric center toward the outer. For this reason, the light intensity for exposing the resist is required greater toward the outer diametrically of the concentric circles. By developing the positive type resist through use of the photomask, a resist pattern is formed having a film thickness decreasing toward the diametric outer.
Meanwhile, JP-A-2003-177507 discloses, as a photomask, a method to form a phase grating with a plurality of grooves on a mask substrate so that the intensity of transmitting light can be changed stepwise depending upon the depth and width of the grooves.
However, in the photomask disclosed in JP-A-H8-166666, the shade areas are arranged concentrically. Due to this, where making a resist film equal in thickness in the positions equal in distant from the center as in the case to form a spherical lens, it can be achieved by concentrically arranging the shade film, i.e. patterning of the photomask. However, in other cases, it is impossible to apply a photomask arranged with shade areas concentrically.
A mask pattern arranged concentric has shade areas arranged in a planar form based on a curve line (hereinafter, referred to as a curve-line pattern).
It is a general practice to perform patterning along the straight lines that are in two directions orthogonal to each other and parallel with the surface of a mask substrate. For this reason, a curve-line pattern, or a pattern based on the other straight lines than in the two directions mutually orthogonal (hereinafter, referred to as an oblique pattern), is formed by a stepwise pattern formed in orthogonal two directions, i.e. in a form combining a multiplicity of rectangles. Thus, for curve-line or oblique patterns, there is a problem that the data required for producing a mask pattern increases in amount proportionally to the number of rectangles combined.
In the photomask disclosed in JP-A-2003-177507, there is a difficulty in precisely regulate the depth of the grooves constituting the phase grating. This raises a problem of a variation in the intensity of transmitting light and hence in resist pattern form.
In the photomask disclosed in JP-A-H8-166666, the resolution in the direction of thickness of the resist relies upon the maximal and minimal values and increment/decrement of the space widths. In the photomask disclosed in JP-A-2003-177507, the resolution relies upon the capability of regulating the depth of the grooves structuring the phase grating. The horizontal resolution relies upon the pitch in mask pattern, in the photomasks disclosed in JP-A-H8-166666 and JP-A-2003-177507. In order to improve the positional accuracy in the horizontal direction or the direction of thickness of the resist, there is a need to further reduce the space width minimal value in the photomask. However, this induces a great rise of photomask manufacturing cost. For this reason, there is a limitation in improving the resist-pattern positional accuracy by the mask pattern alone.